Question

Phenols (ArOH) are relatively acidic, and the presence of a substituent group on the aromatic ring has a large effect. The \(\mathrm{p} K_{\mathrm{a}}\) of unsubstituted phenol, for example, is 9.89, while that of \(p\) -nitrophenol is 7.15. Draw resonance structures of the corresponding phenoxide anions, and explain the data.

Short answer

p-Nitrophenol is more acidic due to better resonance stabilization by the nitro group.

Step-by-step solution

Draw Phenoxide Anion

First, let's consider the phenol molecule. The phenoxide ion forms when phenol loses a proton (H⁺) from its hydroxyl group (OH), creating a negatively charged oxygen (O⁻). The delocalization of this negative charge through resonance is crucial for understanding its stability.

Draw Resonance Structures for Phenoxide

Phenol ionizes to form a phenoxide anion. In phenoxide, the negative charge on the oxygen can resonate into the ring. Draw one structure with the charge on oxygen and the original double bonds on the benzene ring. Then, draw additional structures by moving electrons from double bonds within the ring to create new double bonds, moving the charge around the ring, creating a series of resonance structures.

Analyze p-Nitrophenol Anion

For p-nitrophenol, draw the phenoxide ion similarly as before. Now introduce the nitro group (NO₂) at the para position. The presence of the nitro group, an electron-withdrawing group, stabilizes the phenoxide ion further through additional resonance structures. Draw resonance structures showing the movement of the negative charge from the oxygen to the nitro group, indicating the enhanced delocalization.

Compare Resonance and pKa

Compare the resonance structures of phenoxide from unsubstituted phenol and p-nitrophenol. The nitro group in p-nitrophenol stabilizes the negative charge better due to its electron-withdrawing nature, allowing for more resonance structures. This increased resonance stabilization effectively makes p-nitrophenol more acidic than unsubstituted phenol, as seen by its lower pKa value.

Key concepts

Phenoxide Ion

The phenoxide ion is a key player when studying the acidity of phenols. It forms when a phenol molecule loses a proton from its hydroxyl group (OH), leaving behind an oxygen atom with a negative charge. This negative charge results in the formation of a phenoxide ion, which can be represented as ArO⁻. The stability of the phenoxide ion is crucial because it influences the acidity of the original phenol molecule.
For phenoxide ions, the stability is largely determined by how well the negative charge is distributed across the molecule. This is achieved through resonance, where electrons in the phenoxide ion can shift their positions, spreading the negative charge over the aromatic ring.

• When the negative charge is well-distributed, the ion is more stable, favoring dissociation and thus greater acidity.
• If the charge remains concentrated on the oxygen, the ion is less stable, and the phenol is less acidic.

Understanding phenoxide ions helps explain the acidic nature of phenols and the impact of different substituents on their acidity.

Resonance Structures

Resonance structures are alternate ways of drawing the same molecule, depicting the delocalization of electrons within it. For a phenoxide ion, resonance plays a pivotal role in stabilizing the negative charge created when phenol loses a proton.
When drawing resonance structures, you start with the basic phenoxide form where the negative charge is on the oxygen. Then, you look at how the electrons can move to create new structures. This involves moving one set of electrons at a time from a double bond to the adjacent position, creating new pi bonds and shifting the negative charge. As a result, multiple structures arise, indicating different configurations of electrons.

• These structures are not real, but they help visualize the electron distribution within the molecule.
• The true representation of the molecule is a hybrid of these structures, meaning the negative charge is actually spread across the entire system.

This redistribution of charge through resonance increases the ion's stability and affects how we perceive the compound's reactivity and properties like acidity.

Substituent Effects on Acidity

Substituents on the aromatic ring of phenol dramatically influence its acidity by affecting the stability of the resulting phenoxide ion. Some substituents can either donate or withdraw electrons from the ring, altering the charge distribution and therefore the stability of the phenoxide ion.
An electron-withdrawing group, like a nitro group (NO₂) positioned at the para site, enhances acidity. It pulls electron density away from the ring, facilitating the spread of the negative charge from the phenoxide ion, stabilizing it further.

• This stabilization allows for more resonance structures and a lower pKa value, indicating higher acidity. For example, p-nitrophenol has a pKa of 7.15 compared to 9.89 for unsubstituted phenol.
• In contrast, electron-donating groups can reduce acidity by funneling electron density back into the aromatic system, reducing the negative charge's delocalization.

By understanding these substituent effects, we can predict and explain changes in phenol acidity, crucial for chemical synthesis and reaction predictions.